Old pinball machine PCB resistor missing

Thread Starter

robismod

Joined Sep 22, 2015
125
I have a late 70s pinball machine made by Brunswick (home model) with one PC board mounted behind the back glass. On it there is a resistor that is missing half of it. All that remains are two gold bands. There are no schematics to be found and little other info. No other boards yet to be found...
I’m wanting to hazard a guess at trying some and hope for a good result but don’t have an idea of where to start in values. Anyone take a stab at a guess? (No worries—it’s not a total loss if it don’t work and gets worse)
I’ll see if I can attach a pic of it. (It’s just at the lower left corner of the heat sink.)
Thanks again. 52CF577F-865D-4066-BEBF-842CF975773D.jpeg
 

nsaspook

Joined Aug 27, 2009
7,299
A WAG would be it's part of the uA7815 voltage regulator circuit. I can see some slight residue from what looks like breakage. I've seen carbon resistors crack in half when over-heated. Can we see the back side of that part of the board.
 
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MisterBill2

Joined Jan 23, 2018
6,107
The reasonable way to discover what the value of that resistor would be is to see where in the circuit it is. Certainly, based on the standard color code, it was a low value resistance. Seeing where in the circuit it was located will be needed to avoid just guesses.
Of course another way would be to find the maker of the machine and locate a circuit drawing.
 

Hymie

Joined Mar 30, 2018
819
Before seeing the photo, was expecting to see a burnt up resistor.
Given the lack of heat damage, it looks to me that the machine might have had too much ‘tilt’ action, resulting in mechanical breakage (perhaps due to age).
See Dodgydave’s post above – to be on the safe side start at the high end (maybe 8.2 ohm) – based on its size, it probably had a 1W rating.
 

bertus

Joined Apr 5, 2008
20,540
Hello,

It looks like a mechanical breakdown of the resistor.
Are there no rest pieces of the resistor in the pinball machine found?

Bertus
 

sagor

Joined Mar 10, 2019
207
It also appears to be at least a 1/2W resistor, as well as a low value. If you can figure out that it is part of the 7815 regulator circuit, and try a low value replacement, try to make it a 1W if possible.
If it is part of the 7815, try a low value but measure the current thru it. A 7815, especially with that small heatsink, should not be passing more than 1A, preferably 0.5A (a lot depends on the input voltage).
I've seem some designs where there is a low value resistor on the input side, to create a voltage drop for the regulator when current draw gets higher. This splits the "heat" load between the resistor and the regulator. Usually these are higher wattage resistors (3W or 5W)

Good luck.
 

MrChips

Joined Oct 2, 2009
21,161
As said before, start off with a low value resistor for testing. Two 10Ω ½W resistors in parallel would be a good starting point.
 

MisterBill2

Joined Jan 23, 2018
6,107
Considering the possibility of a resistor in series with the supply to the regulator: I once created a very powerful oscillator by trying to drop about 3 volts with a series resistor in that exact position. So that may not be what it is for. And after a lot of struggling with the photos it sort of looks like it is in series with the input pin of the regulator. What was not clear to me is where the output of the regulator is tied to. It sort of looks like it connects to that string of two small diodes near the heat sink. AND it looks a lot like it has been intentionally unsoldered. So there may be a lot more to the story than what we have heard so far. Such as "other problems not previously mentioned."
I did get a look at that other site and the same PCB, and that resistor is installed the opposite way, and so the two bands visible are gold and gold. So resistor gray red gold gold would be a first choice to try. BUT put it in with longer leads, so that it is a half inch above the board. And check those two small diodes tghat are connected to it, record the numbers because they are zener diodes. a 3-digit number starting with 7 is all I could make out.
 
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Tonyr1084

Joined Sep 24, 2015
4,554
Here's the back side flipped vertically (what appears on the bottom moves to the top) (left stays left and right stays right)
Side by side - the broken resistor is the second solder joint near the mounting hole seen front side, left bottom, from back side, left top. That broken resistor leads back to pin 1 (correct me if I'm wrong) of the 7815 regulator.
1586015897214.png 1586015798410.png
 

Dodgydave

Joined Jun 22, 2012
9,169
So it's in Series with the Input to the regulator, looking at this picture it looks like a 5.6 Ohms..Green Blue Gold.. Bottom RIght.

Thanks to Ian Rogers link.

aspen-brunswick-1979-pinball-machine_1_4572076fd683475784cdaddf17a67b90.jpg
 
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Wolframore

Joined Jan 21, 2019
1,748
It looks like that one has a tendency to burn out... another board that looks repaired.

1586021889087.png1586021921270.png


Here's a manual with a schematic for the machine. unfortunately none for the circuit board, but you may find useful if you don't already have it.
Brunswick Aspen Manual
 

MisterBill2

Joined Jan 23, 2018
6,107
The trick is to install the resistor with longer leads so that the body is at least a quarter inch above the board surface. Not only does that allow more air circulation, but the longer leads also conduct more heat away from the resistor body. AND, what is missing that is very important is the 0.1 mFD capacitors at the regulator, bypassing both the input and output. Those are not optional, they are vital. Without the bypass capacitors the regulator can oscillate very well and get quite hot.
 
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